1. Field of the Invention
The present invention relates to a variable-cycle engine which operates in a two-cycle mode when the rotational speed of the engine is lower than a predetermined speed and in a four-cycle mode when the rotational speed of the engine is higher than the predetermined speed.
2. Description of the Prior Art
Ordinary engines are roughly grouped into two-cycle engines in which the intake, compression, power, and exhaust strokes are performed while the crankshaft makes one revolution and four-cycle engines in which the above four strokes are carried out while the crankshaft makes two revolutions.
In the two-cycle engines, intake ports are positioned in a lower portion of a cylinder liner. Since intake air is drawn and exhaust gases are discharged when the piston is lowered, and the explosion occurs each time the crankshaft makes one revolution, the rotational speed of the output shaft suffers less fluctuations even in a low engine speed range, and the engine can produce a high-torque output.
However, since the intake and exhaust strokes are simultaneously performed in the two-cycle engines, the intake air and the exhaust gases are not fully exchanged particularly in a high engine speed range. Therefore, the two-cycle engines are lower in efficiency and higher in fuel consumption.
In the four-cycle engines, intake air is drawn and exhaust gases are discharged in respective strokes. Therefore, the intake air and the exhaust gases are well exchanged in a high engine speed range. Accordingly, the four-cycle engines are highly efficient, and fuel consumption is lower.
The four-cycle engines suffer larger fluctuations in the rotational speed of the engine output shaft. Therefore, in a low engine speed range, the engine output torque is lower, and the engines do not operate smoothly.